drm/i915/backlight: switch to kernel unsigned int types

We have fairly mixed uintN_t vs. uN usage throughout the driver, but try
to stick to kernel types at least where it's more prevalent.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e68c3f16738eb3ab9f276d797f20326ed6d15848.1528794959.git.jani.nikula@intel.com